Known high pressure pumps for common rail pump systems usually comprise a piston movably supported in a piston bore of a cylinder created in a pump body. The piston is reciprocally driven by an eccentric and alternatingly moves inwards into the piston bore and outwards. In the outwards motion of the piston the piston bore receives fuel from a fuel inlet. In the subsequent inwards motion the piston exerts a certain pressure onto the fuel and provides it to a fuel outlet, which in turn is connected to a rail of a common rail fuel injection system.
The pump body may be attached directly to the engine for easily driving the piston. Usually, the piston is sealed by means of a piston seal and/or a dedicated small clearance relative to the piston bore. Hence, separating fuel and engine oil and the prevention of mixing of engine oil and fuel is an issue when the high pressure pump is mounted on the engine. A reliable separation is desirable for ensuring correct emission norms and avoiding dilution of engine oil due to fuel leakage.